Aneurysms are a fundamental cause of hemorrhagic stroke and accounts for about 20 percent of all stroke cases. If an aneurysm in the brain ruptures, a portion of the brain is filled with blood that can cause tissue death or pressure in the head. Large hemorrhages, generally caused by clearly visible large aneurysm, can also be fatal. A particular case of interest is the debilitating “dementia” like conditions caused by micro hemorrhages that are due to small aneurysm ruptures.
Aneurysms are infrequently encountered on a straight, non-branching segment of an intracranial artery. Aneurysms occurring on straight, non-branching segments are more often found to have sacs that point longitudinally along the walls of the artery in the direction of blood flow, and to project only minimally above the adventitial surface. Aneurysms having these characteristics are of a dissecting type, rather than of a congenital saccular type. The development of dissecting type aneurysms is heralded more frequently by the onset of ischemic neurological deficits than by the subarachnoid hemorrhage associated with congenital saccular aneurysms.
While the underlying mechanisms of aneurysm formation are generally unclear, aneurysms may often develop in association with arterio-venous malformations (“AVM”). AVM's generally consist of snarled tangles of cerebral arteries and veins, and/or spinal arteries and veins. Tangles of arteries and veins lack interconnecting capillary networks necessary to effectively control oxygen delivery to brain tissues.
In addition to oxygen tissue deprivation, rapid arterio-venous flow rates inside the AVM may cause dangerously high blood pressure and vessel wall weakness, potentially leading to vessel deterioration, venous stenosis, aneurysm formation, subsequent hemorrhage, and even stroke. AVM's account for approximately two percent of all hemorrhagic strokes that occur each year, and about one percent of those with AVM's will die as a direct result of AVM's.
Currently, acute stroke diagnosis using computed tomography (“CT”) consists of utilizing non-contrast CT (“CT”) imaging data to rule out cerebral hemorrhage, CT angiography (“CTA”) imaging data to rule out brain aneurysm, dynamic CT perfusion imaging data to assess cerebral perfusion disturbances, and CTA and CT venography (“CTV”) imaging data to rule out AVM's. For large brain coverage, a dynamic CT perfusion scan is generally administered with a volume shuttle protocol or mode. Other acquisition protocols for use in the CT perfusion scan include helical shuttle and dual energy CTA. These dynamic CT perfusion scan protocols have the ability to extract NCT, CTA, and CTV phases from data obtained from a dynamic CT perfusion scan, and thus potentially eliminate additional CTA and CTV scans.
Dual energy CTA provides an ability to separate iodine in a contrast-enhanced vasculature from calcium or bone, thereby removing ambiguities when signal levels between iodine and calcium are comparable. Dual energy CTA also provides an ability to reduce or eliminate beam-hardening effects, which are seen within the cranium.